The present invention relates generally to optoelectronic transceiver modules which utilize semiconductor laser diodes for transmitting data, and in particular to a power stabilizer and a stabilization method for biasing the laser diodes.
Optoelectronic transceiver modules provide an interface between an electrical system and an optical transfer medium such as an optic fiber. Correspondingly, most optoelectronic transceiver modules contain electrical and optical conversion circuitry for transferring data both to and from the electrical system and the optical transfer medium.
Normally, transceiver modules use laser diodes, which produce coherent light, for performing high speed data transfers between the electrical system and the optical transfer medium. Typically, each laser diode is packaged with optical power-monitoring circuitry. For example, the RLD-85PC diode package by ROHM, Inc. contains both a laser diode for transmitting data and a photodiode for performing power-monitoring.
The power-monitoring photodiode within the diode packaging provides a monitor current I.sub.m which varies as the optical power being generated by the laser diode changes. Normally, the changes in the monitor current I.sub.m are directly proportional (i.e., linear) to the changes in the optical power generated by the laser diode. However, the ratio of monitor current I.sub.m with regard to the laser diode's optical power can vary widely from one diode package to the next. Therefore, each diode package must be calibrated separately in order to determine its specific ratio of monitor current I.sub.m to laser diode optical power.
The primary purpose of providing a monitor current I.sub.m is for ensuring that, during operation, the laser diode is within its lasing mode of operation. The minimum current which must be supplied to the laser diode to cause lasing is referred to as the threshold current I.sub.th.
When the current being supplied to the laser diode is less than the required threshold current I.sub.th, the laser diode is said to be operating in the LED mode. In the LED mode, the current supplied to the laser diode is only sufficient enough to excite atoms in the laser diode's cavity which cause light to be emitted in a manner similar to that produced by light emitting diodes (LEDs).
When the current being supplied to the laser diode reaches a level which is either greater than or equal to the threshold current I.sub.th, the laser diode's efficiency of converting electrical current into light will increase dramatically and thus the laser diode changes from the LED mode of operation to the lasing mode of operation.
While various classes of laser diodes will have threshold currents in the same general range, the threshold current I.sub.th can still vary considerably between laser diodes. For example, the threshold current of some types of laser diodes can vary by as much as fifty percent between their typical and maximum values.
Furthermore, when the laser diode is operating in the lasing mode, there is a characteristic slope that is used to determine the laser diode's output efficiency .eta.. As commonly known in the art, the output efficiency .eta. is defined as the ratio of the changing in the laser diode's optical output power in relation to the changing in the operating current while in the lasing mode. However, as with the monitor current I.sub.m, the actual output efficiency .eta. varies from one laser diode to another.
Based on the variance in the monitor current I.sub.m, the threshold current I.sub.th, and output efficiency .eta. of each laser diode, the operating current range for a given laser diode must be calibrated in order to ensure that the laser diode will always be operating within the lasing mode while transmitting data.
The primary method of ensuring that a laser diode will remain in the lasing mode is to provide the diode with a sufficient bias current. In addition, the laser diode is normally supplied with a second signal which is superimposed onto the bias current and corresponds to the data signals to be transmitted. Thus, the data signals are optically transmitted by the modulation of the laser diode's optical power output which is caused by the superimposing of the data signals onto the bias current. Typically, the bias current and the superimposed data signal are generally referred to as the laser diode's operating current I.sub.op.
As indicated previously, great care must be taken to ensure that the maximum variation caused by superimposing the data signals onto the bias current will not cause the laser diode's operating current to fall below the required threshold current level I.sub.th. If the laser diode's operating current falls below the required threshold current level I.sub.th, then as indicated above, a failure to transmit data will occur because the laser diode will revert to the LED mode of operation.
In addition, besides not going below the threshold current level, the operating current must also be maintained at a sufficiently high enough level that a receiving photodiode can detect the modulated light signal. Furthermore, the laser diode's operating current must not be allowed to go so high as to burn out or significantly reduce the useful life of the laser diode.
Normally, transceivers use an analog feedback loop coupled to a mechanical potentiometer for manually adjusting the laser's output power. The optical power is set by adjusting the reference voltage for the analog feedback loop, via the mechanical potentiometer, until the desired amount of optical output power is achieved.
The use of a mechanical potentiometer for setting the output power level presents many problems due to the electrical characteristics of laser diodes. For example, as indicated above, a laser diode will be destroyed if its optical output power exceeds a certain limit. However, accidentally exceeding the laser diode's power limit by trying to set the bias current is generally quite easy since laser diodes typically have a very sharp optical output efficiently slope .eta. once they are in the lasing mode of operation. Thus, losses are commonly caused by adjusting the calibration potentiometer too quickly. Correspondingly, the setup procedure for calibrating laser diodes is generally time consuming and expensive since extreme care must be used in setting the output power via a mechanical potentiometer.
One method proposed for solving the problems of tuning laser diodes is to use a programmed digital controller as set forth by U.S. Pat. No. 5,019,769 which is incorporated herein by reference. The digital controller is used to measure the laser diode's operating characteristics and to control the process of turning on and selecting the operating parameters of the laser diode. However, the use of a digital controller is expensive, consumes additional power, and occupies an inordinate amount of circuit board real estate. Thus, the use of a digital controller is adverse to the wave of inexpensive, low-power, and miniaturized circuitry which is required of today's electrical products.
Furthermore, the use of a digital controller in the control loop (i.e., power stabilizer circuitry) of the laser diode results in adjustments to the laser bias current being made in only certain discrete time intervals, with the time intervals being defined by the operating speed of the digital controller and its software algorithm. Accordingly, the use of a digital controller cannot immediately compensate for power fluctuations which may occur in the optical power output of the laser diode due to power spikes, noise, and other variations in the system.
In view of the above, it is an object of the present invention to provide an optoelectronic transceiver which employs a power stabilization and a stabilization method for efficiently biasing the operating current supplied to a laser diode.
It is another object of the present invention to prevent the destruction of a laser diode during calibration of the output power.
It is still another object of the present invention to provide a cost effective and automated means for selecting the bias current supplied to a laser diode.
A further object of the present invention is to provide a means for biasing a laser diode while minimizing the amount of circuit board space required for such laser diode biasing.
A still further object of the present invention is to provide a stable means for laser diode biasing.
Another object of the present invention is to immediately compensate for power fluctuations which may occur in the optical power output of a laser diode.
Furthermore, other objects, features, and advantages of the present invention will be apparent from the following detailed description taken in connection with the accompanying drawings.